Pacemaker systems with rate control have become widely used in the art. Rate control may be provided by employing one or more rate responsive sensors, e.g., sensors which determine a parameter such as Q-T interval, exercise, etc., from which the desired pacing rate to match the patient's cardiac's demand can be determined. Such rate responsive pacemakers contain algorithms for converting the sensed parameters into pacing rate, e.g., increased activity results in a higher pacing rate. Further, it is known to program certain data relating to pacing rate from an external programmer, e.g., the values of lower rate limit (LRL) and upper rate limit (URL) can be programmed in this manner.
It has been determined that under special circumstances, it is desired to control pacing rate of an implanted pacemaker in accordance with a special function, i.e., at a rate or rates which would not otherwise be indicated. For example, it has been determined that following radio frequency catheter ablation of the atrioventricular junction, there is a certain incidence of ventricular fibrillation or sudden death. See, for example, the article of Peters et al., "Bradycardia Dependent QT Prolongation and Ventricular Fibrillation Following Catheter Ablation of the Atrioventricular Junction With Radiofrequency Energy," PACE, Vol. 17, January 1994; Jordaens et al., "Sudden Death and Long-Term Survival After Ablation of the Atrioventricular Junction," EUR.J.C.P.E., Vol. 3, Nov. 3, 1993; and Geelen et al., "Ventricular Fibrillation and Sudden Death After Radiofrequency Catheter Ablation of the Atrioventricular Junction," PACE, 1996. Indeed, it has been determined that for pacemaker patients with an LRL in the area of 60 bpm, post-ablation there is a risk of about 6% that the patient will develop bradycardia-dependent ventricular fibrillation. In such post-ablation circumstances, the patient's natural fast ventricular rate is replaced by the pacemaker rate. While lower rate pacing does not remove the danger, episodes of ventricular extra-systole (VES) and ventricular tachycardia can be suppressed by overdrive pacing at a higher rate, e.g., 80-90 bpm, or greater. Accordingly, it is known to program a lower rate limit to such a relatively high rate of about 90 bpm, and to then reprogram the lower rate limit back to a more normal rate, e.g., 60 bpm, following a month or so.
However, there remain certain problems with this post-ablation technique. First, the patient comfort may be sacrificed by maintaining the lower rate limit at the constant high rate for too long a period of time. Further, the patient then needs to be re-programmed by the physician, at which time LRL is abruptly dropped to a lower value, e.g., 60 bpm. Further, this procedure provides no flexibility, and does not account for the fact that the high rate overdrive need is not constant, but can be adjusted downward over a time period of approximately a month. Further, the prior art does not take into account the effects of patient exercise. Since the patient remains vulnerable to bradycardia-dependent fibrillation, the rate response during exercise should be adjusted to be more appropriate to this particular situation.
Accordingly, there is a need for a pacemaker system and method for providing special function rate control, to be used for situations such as a post-ablation period or other special diagnosed circumstances where normal rate control is unsatisfactory.